With continuous development of power electronic technologies, as a new-generation direct-current transmission technology, the flexible direct-current transmission technology may resolve many problems existing in current alternating and direct-current transmission technologies. A flexible direct-current transmission system may, together with an alternating-current system, supply power to a user load, so as to improve a power supply capability and power supply reliability. In a normal case, the user load is carried together by the alternating-current transmission system and the flexible direct-current transmission system, and the flexible direct-current transmission system is in an active control mode. When an alternating-current line stops running because of a fault, an island is formed. If there is only one inverter station in the island, the inverter station is in a passive control mode, can independently supply power to a user, and does not depend on the alternating-current system. If there are multiple inverter stations in the island, one of the inverter stations may be selected to enter the passive control mode to establish an alternating-current voltage for the island, and other inverter stations are still in an active control mode. If a fault occurs on the alternating-current line in the island again, multiple islands are formed. Therefore, one inverter station in the newly formed islands enters the passive control mode to establish an alternating-current voltage for the newly formed islands.
Therefore, when an island is formed, an appropriate island detection method is necessarily used to quickly and exactly detect an island state and switch one of the flexible direct-current inverter stations from an active control mode to a passive control mode while preventing other inverter stations from entering the passive control mode. If a fault occurs on the alternating-current transmission line in the island again, multiple islands are formed. Therefore, it needs to be ensured that one flexible direct-current inverter station in the newly formed islands can enter a passive control mode to establish an alternating-current voltage for the newly formed islands and continue supplying power to a user load.
In currently disclosed documents, there is no method for preventing multiple flexible direct-current inverter stations in an island from synchronously entering a passive control mode.